1. Field of the Invention
The present invention relates to a semiconductor laser, especially relates to a semiconductor laser with a grating.
2. Related Prior Art
In the optical fiber communication, particularly in the long reach and high-speed field, semiconductor lasers with a grating has been used. Such semiconductor lasers are a distributed feedback laser (DFB laser), a distributed Bragg reflector laser (DBR laser) and a laser with an external cavity
A configuration and mechanism of the DFB laser are well known to various articles, such as a specification of Japanese Patent laid open 10-051072. The DFB laser provides a light-reflecting surface and a light-emitting surface with relatively low reflectivity to the light-reflective surface. A Bragg grating is disposed between the light-reflecting surface and the light-emitting surface. A relatively high reflective coating on the light-reflecting surface and an anti-reflective coating on the light-emitting surface are disclosed in Japanese Patent laid open 10-190139 and also Japanese Patent laid open 09-326531, respectively.
An optical module including the DFB laser generally provides a thermo-electric element, such as Peltier element, to keep the temperature of the DFB laser constant. In such optical module with the Peltier element, even the ambient temperature change, the practical temperature of the DIB laser may be kept constant through the Peltier element, thereby enabling to output light in single mode.
On the other hand, to install the Peltier element within the optical module results in raising cost of the module due to the increase of parts and assembly process. Similarly, power consumption of the module increases due to current supplied to the Peltier element. The conventional DFB lasers without any Peltier element oscillate in numerous Fabry-Perot mode at low temperatures around −40° C. because the gain attributed to the Fabry-Perot mode superiors to that of the DFB mode at low temperature.